A planetary gear consists of a sun gear, planetary gears, a carrier and an internal gear. In general, planetary gear assemblies are used as decelerators or accelerators. They are geometrically advantageous in that an input shaft and an output shaft are arrayed along the same axis. In a planetary gear assembly, transmission torque is exactly divided among three or four planetary gears which are disposed at rotationally-symmetric positions. Therefore, the transmission efficiency is high, because the torque is equally allocated to the equivalent planetary gears.
In the case of use as a decelerator, an input shaft is connected with a sun gear, and an output shaft is connected with a carrier. An internal gear is fixed to a casing.
In this case, the reduction rate R is given by the equation ##EQU1## where S and I are tooth numbers of the sun gear and the internal gear, respectively.
In most cases like this, planetary gear assemblies are simply used as decelerators with a fixed reduction rate in which the internal gear is fixed.
In a few limited cases, the internal gear is rotatably supported to change the reduction rate continuously. An additional internal gear is shaped on the external surface of the internal gear ring which can rotate along the common central axis. Another adjusting gear (T) meshes with the external gear. In these cases, both sun gear and internal gear supply the input rotation and the carrier gives the output rotation.
The angular velocities of sun gear, internal gear and carrier are denoted by .OMEGA.s, .OMEGA.i and .OMEGA.c respectively. The relation among them is described as EQU S.OMEGA.s+I.OMEGA.i=(S+I).OMEGA.c (2)
The angular velocity .OMEGA.c of the carrier is no longer in proportion to the angular velocity .OMEGA.s of the sun gear. The angular velocity of the carrier can be adjusted by changing the velocity .OMEGA.i of the internal gear.
The tooth number of the adjusting gear (T) is denoted by T. Its angular velocity is denoted by .OMEGA.t. The adjusting gear (T) meshes with the external gear whose tooth number is E. The external gear (E) and the internal gear (I) are shaped on opposing surfaces of the same ring. The restriction in the meshment is expressed by EQU E.OMEGA.i=-T.OMEGA.t (3)
Substituting Eq. (3) into Eq. (2), we obtain the equation ##EQU2## This means that the angular velocity .OMEGA. c of the carrier can be changed by the rotation of the adjusting gear (T) even if the angular velocity of the sun gear is kept constant.
In these cases, the internal gear is positively and definitely rotated by the adjusting gear for controlling the rotation of the carrier. The ring on which the internal gear and the external gear are shaped is called an "internally-toothed ring gear." The outer ring must be rotatably supported by a bearing. The supporting surfaces of the bearing are side cylindrical surfaces whose diameters are smaller than that of the pitch circle of the external gear of the outer ring. The external gear is shaped between two side supporting surfaces.
Besides the reduction rate adjustable planetary gear assemblies above-mentioned, some kinds of planetary gear assemblies have internal gears which are rotatably sustained in a casing to alleviate an external shock. In this case, the internally-toothed ring gear is elastically sustained by a casing. For example, the casing can have several inner projections and the internally-toothed ring gear can have several outer projections. The inner projections of the casing mesh with the outer projections of the internally-toothed ring gear with an elastic material such as rubber or plastic disposed in the space between the outer and inner projections. In a normal state, where transmission torque is under a determined value, the internally-toothed ring gear is fixed. When an excess of torque is applied to the output shaft, the internally-toothed ring gear rotates slightly to alleviate the external shock (i.e., excess of torque). These types of gears are described in British Patent Application 2,107,425 and British Patent 1,116,791.
For the purpose of absorbing an excess of torque, the internally-toothed ring gear need not be sustained by a bearing, because the internally-toothed ring gear rotates by only a very small angle at the application of the external shock. The internally-toothed ring gear has no external gear in the outer surface, because the internal gear does not rotate positively.
Thus, there have been two types of planetary gear devices in which the internally-toothed ring gear rotates to any degree. One is a device for controlling the reduction rate by positively rotating the internal gear. The other is a device for alleviating external shock by permitting the internal gear to rotate slightly.
In either case, the internally-toothed ring gear does not rotate freely. Free rotation of the internal gear is forbidden. Here, "free rotation" means an arbitrary rotation angle and an arbitrary rotation velocity.
In the case of a reduction rate adjustable device, the angular velocity of the internally-toothed ring gear is definitely controlled by an additional gear. In the case of a shock absorbing device, the internally-toothed ring gear rotates very little. The rotation angle of the internally-toothed ring gear is far less than 90 degrees. In most cases, it is less than 10 degrees.
Both improved devices permit the internal gear to rotate by any means to give the carrier a torque output. There has never been a planetary gear assembly which gives the carrier a torque output at some times but gives it no torque output at other times.